Because nitride semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN), gallium aluminum nitride (GaAlN), etc. have sufficiently wide forbidden bands with direct interband transition, their applications to short-wavelength light-emitting devices have been investigated. In addition, because of a large saturated drift velocity of electrons and the usability of a two-dimensional carrier gas in heterojunctions, their applications to electronic devices are expected.
Base substrates for the growth of GaN widely used at present are made of sapphire, and devices are generally produced by the following method. Namely, GaN is heteroepitaxially grown on a single-crystal sapphire base substrate by vapor phase growth methods such as a metal-organic vapor phase epitaxy (MOVPE) method, a molecular beam epitaxy (MBE) method, a hydride vapor phase epitaxy (HVPE) method, etc., and an epitaxial layer of a nitride semiconductor is grown thereon continuously in the same chamber or after transferred to a different chamber.
Because a sapphire base substrate and GaN are different in lattice constant, the growth of GaN directly on the sapphire base substrate fails to provide a single-crystal layer. Thus, JP 4-297023 A proposes the application of a low-temperature buffer layer technology developed for the purpose of the heterogrowth of Si, etc. on a sapphire base substrate to the growth of GaN, providing a method of growing GaN on a sapphire base substrate at as low a temperature as about 500° C., such that the resultant low-temperature-grown buffer layer relaxes lattice strain, and growing GaN thereon. The use of a low-temperature-grown nitride layer as a low-temperature buffer layer enables the epitaxial growth of single-crystal GaN on the sapphire base substrate. However, because there is a narrow optimum range between temperature and thickness in the growth of the low-temperature buffer layer, it is difficult to form a low-temperature buffer layer with good reproducibility.
Thus, instead of continuously conducting the epitaxial growth of a multilayer semiconductor film having a device structure on a sapphire base substrate, there is a method of growing a GaN layer on a base substrate in advance, and growing an epitaxial layer on the resultant substrate to form a device.
With a so-called GaN template provided by growing a GaN layer on the sapphire base substrate, however, even the low-temperature buffer layer technology cannot remove the discrepancy of a lattice between a substrate and a crystal, resulting in a GaN layer having a dislocation density of about 109 to 1010/cm2. Because this defect obstructs the production of GaN devices, particularly laser diodes (LDs) and ultraviolet light-emitting diodes (LEDs), GaN templates are predominantly used for visible-ray light-emitting diodes (LEDs) and electronic devices having characteristics little affected by dislocation.
In the case of LDs and ultraviolet LEDs needing epitaxial layers having low dislocation densities, a method of using a substrate constituted only by a GaN layer as a substrate for crystal growth, and forming a multilayer semiconductor film for constituting device elements on the substrate has been considered. Such a substrate constituted only by GaN for crystal growth is called “self-supported GaN substrate.” Though the crystal growth of nitride semiconductors in a bulk form has been difficult, the development of self-supported GaN substrates on a practically usable level has recently been succeeded at last.
The self-supported GaN substrate is generally obtained by epitaxial growth of a thick GaN layer having a low dislocation density on a different base substrate such as a sapphire base substrate, and then separating the GaN layer from the base substrate. For instance, JP 10-256662 A discloses a method for growing a thick GaN layer on a sapphire base substrate, and then separating the GaN layer from the sapphire base substrate. JP 11-251253 A discloses a method for producing a self-supported GaN substrate comprising forming a GaN layer on sapphire base substrate, using a dislocation-reducing technology called “epitaxial lateral overgrowth (ELO)” described in Appl. Phys. Lett., Vol. 71 No. 18 (1997) p 2638, and etching the sapphire base substrate for removal. JP 2003-178984 A discloses a method comprising growing a GaN layer having a low dislocation density on a base substrate of sapphire, etc. via a thin TiN film having a network structure, using a void-assisted separation (VAS) method described in Y. Oshima, et al., Jpn. J. Appl. Phys., Vol. 42 (2003) pp. L1 to L3, and easily separating the GaN layer from the base substrate by voids in their interface. In an as-grown state, the GaN substrate obtained by these methods usually has morphology such as pits, hillocks, etc. on its surface, resulting in difficulty in growing an epitaxial layer for producing devices without further treatment. Therefore, the top surface of the substrate is generally mirror-polished before devices are produced thereon.
An MOVPE method is often used to epitaxially grow a group III nitride semiconductor having a device structure. When a group III nitride semiconductor is epitaxially grown on a sapphire base substrate by the MOVPE method, a so-called thermal cleaning, by which the sapphire base substrate is heated at 1000° C. or higher for a certain period of time in a hydrogen gas atmosphere, is generally carried out to remove stain from the sapphire base substrate surface.
When a group III nitride semiconductor single crystal is epitaxially grown on a GaN substrate (GaN template or self-supported GaN substrate) by the MOVPE method, too, the thermal cleaning is carried out to remove stain from the GaN substrate surface, or to remove residual strain added at the time of mirror finishing. For instance, JP 2000-252217 A discloses a method for removing defects from a ground surface of a single-crystal GaN substrate by heating the GaN substrate in an atmosphere containing hydrogen and ammonia before epitaxial growth. Also, JP 2003-59835 A discloses a method for carrying out the thermal cleaning of a GaN template or a self-supported GaN substrate at a temperature of 1200° C. or lower.
When the GaN template or the self-supported GaN substrate is subjected to thermal cleaning, it is desirable to remove only stain or a strain layer from a substrate surface, such that the GaN crystal surface is not damaged. However, surface roughening, a phenomenon that a GaN crystal on the substrate surface is thermally decomposed or sublimed, so that the flatness of the substrate surface is deteriorated, may likely occur on the substrate during the thermal cleaning. In the case of thermal decomposition, metallic Ga droplets may be formed on the substrate surface. Thermally decomposed or sublimed GaN may be regrown on the substrate at different points, deteriorating single crystallinity. In any case, the resultant surface roughness of the substrate disturbs interface sharpness with a grown epitaxial layer, and thus the crystallinity and flatness of the epitaxial layer, resulting in the deterioration of the characteristics and reliability of the resultant devices.
The thermal decomposition or sublimation of a GaN crystal depends not only on a temperature, but also on the type and pressure of a gas during the thermal treatment. The method of JP 2003-59835 A is based on the concept that thermal cleaning is conducted without damaging the substrate surface, thereby starting crystal growth before surface roughening occurs on the substrate by heating. Even when thermal cleaning is conducted under the same conditions, however, this method causes surface roughening on some substrates, thereby being unsatisfactory in reproducibility.
A production lot of semiconductor substrates of GaAs, etc. are usually constituted by wafers cut out from the same bulk crystal, resulting in little unevenness in the characteristics of wafers in the lot. However, the GaN substrates are likelier to have uneven wafer characteristics than other semiconductor substrates of GaAs, etc., even in the same lot, because crystal is grown one by one as described above in the case of the GaN substrates. Accordingly, there is a problem peculiar to the GaN substrates that even when a plurality of GaN substrates (wafers) taken from the same lot are subjected to thermal cleaning under the same conditions, some substrates may suffer from surface roughness.